Fossil fuels have had limited use as the energy source for turbines due to particulate matter that is produced in their combustion. This matter can cause the erosion and destruction of the turbine blades. At the high temperature required for efficient operation, often in excess of 900.degree. C., deterioration of blades may be rapid resulting in down time and expensive repairs.
One method of overcoming this problem is to utilize heat exchangers where the combustion gases are passed through a system that utilizes a secondary flow path where "clean" air is separated by plates or tubes through which the heat is transferred. The clean air is then utilized to drive the turbine blades. The use of heat exchangers to drive turbines has been limited due to the lack of suitable materials and the inherent loss of efficiency in transferring heat.
A second approach to this problem has been to filter the combustion gas, removing the particulate matter. While some success has been achieved, only limited use has developed due to the lack of suitable materials to construct filter tubes that have sufficient permeability and high temperature strength.
Extensive work has been done in testing materials and designing hot gas filtration systems for this purpose. One material which has shown promise is clay-bonded silicon carbide which has good chemical resistance and good physical properties at elevated temperatures. However, this material has limits in that it loses strength at very high temperatures and oxidation produces a glass formation that decreases permeability and can seriously restrict filtration. High temperature creep also deforms and damages these filter tubes.
Recrystallized silicon carbide has been extensively used for kiln furniture and structural shapes in high temperature applications. This type of silicon carbide has much higher strength, both at room temperature and elevated temperature, than clay bonded silicon carbide. However, commercial recrystallized silicon carbide is produced with the highest density possible to provide the maximum achievable strength for its normal use as a structural refractory. This high density restricts both the pore size and the permeability of the material making it unsuitable for a gas filter.
The unique refractory product of the present invention, however, along with the method for making the product, resolves the problems indicated above and provides other features and advantages heretofore not obtainable.